What indicates the aircraft requires immediate attention due to fire?

The indication that the aircraft requires immediate attention due to fire is represented by the presence of an engine fire. This situation is critical because an engine fire poses a significant hazard not only to the aircraft and its systems but also to the safety of those onboard and on the ground. An engine fire must be addressed without delay, as it can lead to catastrophic consequences if not extinguished promptly.

When a fire is detected in the engine, it necessitates immediate action according to established safety protocols. Firefighting measures, such as activating fire suppression systems, evacuating personnel if necessary, and alerting emergency services, are essential steps in managing this urgent situation.

The other options do not indicate an immediate fire threat. An engine running, for example, does not automatically mean there is a fire; it is a normal operating condition. Requests for wingwalkers pertain to ground movement safety rather than fire emergencies. The deactivation of an airstart unit is associated with engine starting procedures and doesn't relate directly to the presence of flame or combustion hazards. Hence, when identifying what signals an urgent need for firefighting action, the detection of an engine fire is unequivocally the most critical indicator.